The process herein relates to reducing the nitrogen content of shale oil by contacting the shale oil with a substantially immiscible solvent.
The term "oil shale" as used in the industry is a sedimentary formation comprising marlstone deposits with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. The formation containing kerogen is called "oil shale" herein and the liquid product produced upon decomposition of kerogen is called "shale oil".
In a preferred practice of the method described herein, the method is utilized for refining shale oil produced from in situ retorting of oil shale. An in situ oil shale retort can be formed by many methods, such as the methods disclosed in U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; and 4,043,598, all of which are incorporated herein by this reference.
Although the process disclosed herein of reducing the nitrogen content of shale oil is primarily discussed in relation to shale oil produced from the in situ retorting of oil shale, the process can be practiced on shale oil produced by other methods of retorting. Many of these methods for shale oil production are described in Synthetic Fuels Data Handbook, compiled by Dr. Thomas A. Hendrickson, and published by Cameron Engineers, Inc., Denver, Colo. For example, other processes for retorting oil shale include those known as the TOSCO, Paraho Direct, Paraho Indirect, N-T-U, and Bureau of Mines, Rock Springs, processes.
Kerogen is considered to have been formed by the deposition of plant and animal remains in marine and nonmarine environments. Its formation is unique in nature. Alteration of this deposited material during subsequent geological periods produced a wide variety of organic materials. Source material and conditions of deposition were major factors influencing the type of final product formed.
Kerogen samples, found in various parts of the world, have nearly the same elemental composition. However, kerogen can consist of many different compounds having differing chemical structures. Some compounds found in kerogen have the structures of proteins while some have structures of terpenoids, and others have structures of asphalts and bitumens.
Shale oils are generally high molecular weight, viscous organic liquids, of predominantly hydrocarbonaceous oxygen, nitrogen and sulfur-containing organic compounds produced from oil shale. The shale oils are of varying linear, branched cyclic, aromatic hydrocarbon and substituted hydrocarbon content with high pour points, moderate sulfur content and relatively high nitrogen content. The shale oil produced from an oil shale formation can vary between strata within the oil shale formation. The nitrogen content of shale oil can also vary dependent upon the geographical location of the oil shale deposit from which the shale oil is produced. Such a variance in nitrogen content in different geographical locations can be attributed to differences in the environment during the time of the deposition of the organisms which, upon lithification, become oil shale. Such a variance can also be attributed to the different types of organisms in the separate geographical locations which were deposited to form the organic substance in the oil shale and any organisms within the formed deposit layer which acted upon such deposited material to provide the kerogen within the oil shale formation. Furthermore, the nitrogen content of shale oil may vary according to the process and operating variables used to produce it.
The nitrogen content in shale oil is attributable to basic nitrogen-containing compounds and non-basic nitrogen-containing compounds. The relative percentages of the basic and non-basic nitrogen compounds comprising the total nitrogen content of a shale oil varies depending upon the particular shale oil but typically are in the ranges of 60% to 70% basic nitrogen-containing compounds and 30% to 40% non-basic nitrogen-containing compounds.
The nitrogen content of shale oil is generally up to about two percent by weight. For example, the average nitrogen content of shale oil recovered by in situ retorting of oil shale from the Piceance Creek Basin of Western Colorado is on the order of about 1.4% by weight. This is very high when compared with the nitrogen content of crude petroleum which is typically up to about 0.2% by weight.
The presence of nitrogen in shale oil presents many problems in that the nitrogen can interfere with the transportation and use of the shale oil. Deleterious effects brought about by the presence of nitrogen in shale oil are decreased catalyst life in hydrogenation, reforming, hydrocracking and catalytic cracking reactions, decreased chemical stability of products, and decreased color stability of products. Another problem with the presence of nitrogen in shale oil is that it is undesirable to transport nitrogen-containing shale oil through pipelines which are also used for transporting petroleum products because of possible contamination of such products with residual nitrogen-containing shale oil in the pipeline. Generally such petroleum products contain a very low nitrogen content. The relatively high nitrogen content in the shale oil can pollute the pipelines making them undesirable and uneconomical for transporting such low nitrogen-containing petroleum products. In addition, high nitrogen content in shale oil can cause clogging of pipelines due to self-polymerization brought about by the reactivity of the nitrogen-containing compounds. Due to the basicity of the nitrogen-containing compounds in shale oil, some corrosion can occur, thus damaging a pipeline used to transport shale oil.
Product stability is a problem that is common to many products derived from shale oil with the major exception of the asphalt cut and those products that have undergone extensive hydrotreating. Such instability, including photosensitivity, is believed to result primarily from the presence of nitrogen-containing compounds.
It is, therefore, desirable to reduce the nitrogen content of shale oil to increase the utility, transportability, and stability of the shale oil and the products derived from such shale oil.
Due to the undesirable nature of nitrogen in organic fluid streams, such as fluid streams produced in the recovery and refining of petroleum, coal and oil shale, many processes have been developed to reduce the nitrogen content to an acceptable level. The level of acceptability for the nitrogen content is generally based upon the use of the particular stream.
In U.S. Pat. No. 3,719,587 to Karchmer et al, a process is disclosed for removing basic nitrogen-containing compounds from coal naphtha. The basic nitrogen compounds are removed by washing the naphtha with water or with a dilute aqueous solution of an acid. The dilute acid solutions are disclosed as sulfuric acid, hydrochloric acid, phosphoric acid and acetic acid.
U.S. Pat. No. 2,848,375 to Gatsis discloses a process for removing basic nitrogen compounds from organic substances by washing with a weak acid in combination with a polyalcohol. The weak acid used is boric acid in combination with a polyhydroxy organic compound which has hydroxyl groups on adjacent carbons.
U.S. Pat. No. 2,035,583 to Bailey discloses a process for the separation and recovery of nitrogen bases from mineral oils. In the process, the mineral oil is extracted with a solvent for the nitrogen bases. Acceptable solvents are liquid sulfur dioxide, furfural, aniline, nitrobenzene and isobutyl alcohol. However, due to the solubility of desirable mineral oils, such as aromatics and olefines, the process also includes extracting the resultant extract with dilute aqueous acids to recover the nitrogen bases from the first extract. The nitrogen bases are then recovered from the aqueous solution by adding an inorganic base to precipitate the nitrogen bases.
U.S. Pat. No. 2,035,012 to Stratford et al discloses a process for improving the color and viscosity of petroleum oils. In the process an oil is extracted with a selective solvent in combination with an acid. The selective solvent can be phenol, nitrobenzene, furfural or liquid sulfur dioxide. The acid is preferably an inorganic acid but can also be an organic acid such as picric, acetic, oxalic, and citric acids.
U.S. Pat. No. 2,541,458 to Berg discloses a process for recovery of nitrogen bases from hydrocarbon fractions. In the process the fraction is extracted with a volatile acid or non-volatile acid salt in combination with a mutual solvent for the acid and the hydrocarbon fraction. The mutual solvents include low boiling alcohols and ketones. The extraction is conducted in the presence of water to avoid loss of the volatile acids.
U.S. Pat. No. 2,309,324 to McAllister et al discloses a method for removing nitrogen bases from water-insoluable organic solvents, mineral oils and hydrocarbon fractions. In the process the mineral oil is extracted with an aqueous, weak acid solution. The weak acids are classified as acids having dissociation constants below 10.sup.-3. The aqueous acid solutions are prepared by dissolving from 15 to 90 weight percent of an acid in water. Upon extraction of the oil, two phases are formed. The aqueous phase contains the acid and absorbed nitrogen bases. The other phase consists of the organic substance from which at least a portion of the nitrogen bases has been removed.
U.S. Pat. No. 2,662,843 to Castner et al discloses a method to reduce the nitrogen content of shale oil by contacting the shale oil with from two to ten stoichiometric proportions of formic acid for each proportion of nitrogen present in the shale oil.
U.S. Pat. No. 4,071,435 to Smith describes a process for reducing the nitrogen content of syncrude feed wherein syncrude feed is contacted with an aqueous solution containing an extractant-catalyst which is effective for extracting nitrogen-containing compounds from the syncrude feed and for serving as a hydrocracking catalyt for the syncrude feed. Extractant-catalysts include halides of zinc, cadmium, mercury, lead and copper.
U.S. Pat. No. 4,272,361 to Compton discloses a method for reducing the nitrogen content of shale oil by contact with a solvent comprising an active solvent for nitrogen-containing compounds and sufficient water to provide phase separation. The active solvent is selected from the group consisting of organic acids and substituted organic acids.
U.S. Pat. No. 4,209,385 to Stover discloses a method for reducing the nitrogen content of shale oil wherein the shale oil is contacted with a mixture comprising an organic acid and a mineral acid. The organic acid is selected from organic acids and substituted organic acids. The mineral acid is selected from the group consisting of hydrochloric acid, nitric acid, nitrous acid, sulfuric acid, sulfurous acid, phosphorus acid, and mixtures thereof.
U.S. Pat. No. 4,261,813 to Smith describes a method for denitrogenating an oil having a high nitrogen content wherein nitrogenous compounds are extracted with an operable acid solvent. Preferred solvents comprise phosphoric acid, formic acid and the monoammonium salt of phosphoric acid.
U.S. Pat. No. 4,271,009 to Stover discloses a method for reducing the nitrogen content of shale oil which comprises lowering the viscosity of the shale oil and then contacting the shale oil with a solvent comprising water and an organic acid selected from the group consisting of organic acids and substituted organic acids.
U.S. Pat. No. 4,231,858 discloses a method of reducing the nitrogen content of shale oil wherein crude shale oil is contacted with a catalyst and hydrogen at a temperature of about 600.degree. F. and a pressure of about 200 psig to about 5000 psig until a substantial amount of nitrogen-containing compounds is converted to basic nitrogen compounds. The basic nitrogen compounds are contacted with dry hydrochloric acid and the reaction product is removed from the oil.
Many of the processes described in the above patents do not address themselves to the removal of non-basic nitrogen compounds which can be present in organic fluids. Additionally, many of the above-described processes are not specific for treatment of shale oil and the relatively high nitrogen content found in shale oil.
In processes capable of acid solvent extraction of nitrogen-containing compounds, a portion of the hydrocarbons in the shale oil are removed. Increased concentration of the acid solvent removes greater amounts of nitrogen-containing compounds but also extracts greater amounts of hydrocarbons, thereby reducing the hydrocarbon recovery. Accordingly, it is desirable to improve the selectivity of a process towards the removal of nitrogen-containing compounds to thereby provide a process capable of high nitrogen-containing compound extraction along with high hydrocarbon recovery.